Apoptosis is the best-characterized mode of physiological cell death, which plays an essential role in the development and homeostasis of multicellular organisms. Apoptosis is executed by caspases, a family of cysteine proteases, whose activation is initiated via two major pathways: the death receptor (extrinsic) pathway and the mitochondrial (intrinsic) pathway. The activated caspases cleave a number of cellular proteins to generate many of the hallmark morphological features of apoptosis, including DNA fragmentation and membrane blebbing.
The Bcl-2 family of proteins plays a pivotal role in apoptosis by regulating the mitochondrial outer membrane permeabilization (MOMP). MOMP results in the release of apoptogenic factors (e.g., cytochrome c and Smac) from the mitochondria into the cytosol where they directly promote caspase activation and subsequent cell death. Members of the Bcl-2 family contain up to four evolutionarily conserved domains called Bcl-2 homology (BH) domains 1 to 4 and can be classified into three groups based on their domain architecture and function in apoptosis: multidomain (BH1-4) anti-apoptotic Bcl-2 proteins (e.g., Bcl-2, Bcl-XL and Mcl-1), multidomain (BH1-3) pro-apoptotic Bcl-2 proteins (e.g., Bax and Bak), and BH3-only Bcl-2 proteins (e.g., Bad, Bid, Bim, Noxa and Puma). Many of the Bcl-2 family proteins can interact with each other to determine the cell fate. The three-dimensional structures reveal that the BH1-3 domains of anti-apoptotic Bcl-2 proteins form a hydrophobic surface groove to which the BH3 domains of pro-apoptotic Bcl-2 family members bind (Sattler et al., (1997) Science 275:983-986; Day et al., (2008) J Mol Biol 380:958-971). The multidomain pro-apoptotic Bcl-2 proteins Bax and Bak are two major effectors of MOMP, which homo-oligomerize and form pores in the mitochondrial outer membrane to induce MOMP upon apoptotic stimulation. The anti-apoptotic Bcl-2 proteins prevent MOMP by directly binding to both classes of pro-apoptotic Bcl-2 proteins. In contrast, the BH3-only proteins trigger Bax and Bak to induce MOMP. Based on their ability to interact with the multidomain anti- and pro-apoptotic Bcl-2 proteins, the BH3-only proteins are often further divided into two subgroups: direct activators and sensitizers/de-repressors. The direct activators, including Bid, Bim and Puma, are able to not only interact with and inhibit all the anti-apoptotic Bcl-2 proteins but also directly bind to and activate the effectors Bax and Bak. On the other hand, the sensitizers/de-repressors appear to function essentially as transdominant inhibitors by occupying the hydrophobic groove of anti-apoptotic Bcl-2 proteins, thereby displacing the direct activators to promote MOMP and preventing any future bindings of the direct activators or effectors to anti-apoptotic Bcl-2 proteins. Moreover, unlike the direct activators, the sensitizers/de-repressors are more selective in binding to the anti-apoptotic Bcl-2 members. For example, Bad binds and antagonizes Bcl-2 and Bcl-XL but not Mcl-1, whereas Noxa binds and antagonizes Mcl-1 but not Bcl-2 and Bcl-XL. This observation indicates that the BH3-only proteins provide a fine control of MOMP in a Bax/Bak-dependent manner and opportunities to design specific inhibitors for each of the anti-apoptotic Bcl-2 family members.
The evasion of apoptosis is considered to be a hallmark of cancers and a cause of resistance to radiation and chemotherapies. Consistently, high levels of the anti-apoptotic Bcl-2 family proteins are associated with the pathogenesis of cancer and resistance to therapy (Reed et al., (1996) J Cell Biochem 60:23-32; Reed, (1997) Adv Pharmacol 41:501-532). A recent analysis of somatic copy-number alterations (SCNAs) showed that two anti-apoptotic Bcl-2 family genes (Bcl-XL and Mcl-1) undergo frequent somatic amplifications in multiple cancers and that cancer cells carrying Bcl-XL and Mcl-1 amplifications are dependent on the expression of these genes for survival (Beroukhim et al., (2010) Nature 463:899-905). Thus, Bcl-XL and Mcl-1 are very attractive targets for the development of anticancer agents.
Over the last few years, several small-molecule Bcl-2 inhibitors have been synthesized as BH3 mimetics and some of these molecules have entered clinical trials (Yip et al., (2008) Oncogene 27:6398-6406; Vogler et al., (2009) Cell Death Differ 16:360-367; Kazi et al., (2011) J Biol Chem 286:9382-9392). Although Bcl-2 and Bcl-XL have been the primary focus for the design of small-molecule inhibitors, recent studies have demonstrated that Mcl-1 also plays an important role for cancer cell survival and that it is necessary to neutralize both arms of the anti-apoptotic Bcl-2 family (Bcl-2/Bcl-XL and Mcl-1) for apoptosis to occur in many cell types (Willis et al., (2005) Genes Dev 19:1294-1305).
To date, the most potent and selective small-molecule Bcl-2 inhibitors are ABT-737 and its orally active analog ABT-263, which inhibit Bcl-2 and Bcl-XL at subnanomolar concentrations but only weakly target Mcl-1 (Tse et al., (2008) Cancer Res 68:3421-3428). Consequently, these agents generally lack efficacy in cancers with elevated Mcl-1 and in many instances this resistance can be overcome by downregulation of Mcl-1 (Id.; Oltersdorf et al., (2005) Nature 435:677-681; van Delft et al., (2006) Cancer Cell 10:389-399; Chen et al., (2007) Cancer Res 67, 782-791; Konopleva et al., (2006) Cancer Cell 10:375-388; Lin et al., (2007) Oncogene 26:3972-3979; Tahir et al., (2007) Cancer Res 67:1176-1183). Moreover, it has recently been shown that cancer cells can quickly acquire resistance to ABT-737 by upregulation of Mcl-1 (Yecies et al., (2009) Blood 233-304; Hikita et al., (2010) Hepatology 52:1310-1321). What are thus needed are compounds that specifically bind to Mcl-1 to overcome such resistance. Such compounds can be used to treat and/or prevent various cancers. The compositions and methods disclosed herein address these and other needs.